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Saturday, 17 January 2015

Motor control circuits

P L C based Process Control

The interlock contacts installed in the previous section's motor control
circuit work fine, but the motor will run only as long as each
push button switch is held down. If we wanted to keep the motor running
even after the operator takes his or her hand off the control
switch(es), we could change the circuit in a couple of different ways:
we could replace the push button switches with toggle switches, or we
could add some more relay logic to "latch" the control circuit with a
single, momentary actuation of either switch. Let's see how the second
approach is implemented, since it is commonly used in industry:

When the "Forward" push button is actuated, M1 will energize,
closing the normally-open auxiliary contact in parallel with that
switch. When the push button is released, the closed M1 auxiliary contact will maintain current to the coil of M1,
thus latching the "Forward" circuit in the "on" state. The same sort
of thing will happen when the "Reverse" push button is pressed. These
parallel auxiliary contacts are sometimes referred to as seal-in contacts, the word "seal" meaning essentially the same thing as the word latch.
However, this creates a new problem: how to stop the motor! As
the circuit exists right now, the motor will run either forward or
backward once the corresponding push button switch is pressed, and will
continue to run as long as there is power. To stop either circuit
(forward or backward), we require some means for the operator to
interrupt power to the motor contactors. We'll call this new switch, Stop:
Now, if either forward or reverse circuits are latched, they may be
"unlatched" by momentarily pressing the "Stop" push button, which will
open either forward or reverse circuit, DE-energizing the energized
contactor, and returning the seal-in contact to its normal (open) state.
The "Stop" switch, having normally-closed contacts, will conduct power
to either forward or reverse circuits when released.
So far, so good. Let's consider another practical aspect of our motor
control scheme before we quit adding to it. If our hypothetical motor
turned a mechanical load with a lot of momentum, such as a large air
fan, the motor might continue to coast for a substantial amount of time
after the stop button had been pressed. This could be problematic if an
operator were to try to reverse the motor direction without waiting for
the fan to stop turning. If the fan was still coasting forward and the
"Reverse" push button was pressed, the motor would struggle to overcome
that inertia of the large fan as it tried to begin turning in reverse,
drawing excessive current and potentially reducing the life of the
motor, drive mechanisms, and fan. What we might like to have is some
kind of a time-delay function in this motor control system to prevent
such a premature start up from happening.
Let's begin by adding a couple of time-delay relay coils, one in
parallel with each motor contactor coil. If we use contacts that delay
returning to their normal state, these relays will provide us a "memory"
of which direction the motor was last powered to turn. What we want
each time-delay contact to do is to open the starting-switch leg of the
opposite rotation circuit for several seconds, while the fan coasts to a
halt.
If the motor has been running in the forward direction, both M1 and TD1 will have been energized. This being the case, the normally-closed, timed-closed contact of TD1 between wires 8 and 5 will have immediately opened the moment TD1 was energized. When the stop button is pressed, contact TD1
waits for the specified amount of time before returning to its
normally-closed state, thus holding the reverse push button circuit open
for the duration so M2 can't be energized. When TD1 times out, the contact will close and the circuit will allow M2 to be energized, if the reverse push button is pressed. In like manner, TD2 will prevent the "Forward" push button from energizing M1 until the prescribed time delay after M2 (and TD2) have been DE-energized.
The careful observer will notice that the time-interlocking functions of TD1 and TD2 render the M1 and M2 interlocking contacts redundant. We can get rid of auxiliary contacts M1 and M2 for interlocks and just use TD1 and TD2's
contacts, since they immediately open when their respective relay coils
are energized, thus "locking out" one contactor if the other is
energized. Each time delay relay will serve a dual purpose: preventing
the other contactor from energizing while the motor is running, and
preventing the same contactor from energizing until a prescribed time
after motor shutdown. The resulting circuit has the advantage of being
simpler than the previous example:
REVIEW:

Continuous motor operation with a momentary "start" switch is
possible if a normally-open "seal-in" contact from the contactor is
connected in parallel with the start switch, so that once the contactor
is energized it maintains power to itself and keeps itself "latched" on.

Time delay relays are commonly used in large motor control circuits
to prevent the motor from being started (or reversed) until a certain
amount of time has elapsed from an event.